1. Field of the Invention
The present invention relates to a low noise multi-stage type amplifier used in, e.g., a direct satellite (DBS) system.
2. Related Background Art
The NF (noise factor) of a multistage amplifier constituted by a plurality of FETs (field effect transistors) is changed depending on a drain current flowing through a drain-source path of each FET. For this reason, the operation current of each FET is set to have a drain current value for minimizing the NF.
However, since FETs suffer from manufacturing variations, threshold value voltages of FETs also vary, and as a result, operation current values vary accordingly. Therefore, even when the operation current value of the multistage amplifier is set to be a value for minimizing the NF like in the conventional amplifier, the NF cannot be stabilized due to a variation in operation current value caused by manufacturing variations of FETs, and the characteristics of the amplifier become unstable.
As a conventional multistage amplifier of this type, for example, an amplifier described in "X-Band Monolithic Series Feedback LNA", IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. MTI-33, NO. 12, Dec. 1985 is known. This reference discloses a multistage amplifier having a three-stage structure using FETs each having a gate length of 0.5 .mu.m, and a gate width of 300 .mu.m.
However, in the conventional multistage amplifier, a stub to be added to the source of an FET in each stage, and an inter-stage matching network are not optimally designed. More specifically, the characteristics of the amplifier are changed due to manufacturing variations of FETs, resulting in poor I/O impedance matching of the amplifier.